German patent application DE 10 2011 101 868 A1 describes a device for varying the charge-exchange valve stroke, whereby a cam segment that supports a cam group with cams having different stroke contours is arranged so as to be axially movable on a base shaft. The cam segment and the base shaft are splined with each other so that a torque-proof coupling between the camshaft and the cam segment is ensured and the cam segments can move axially via a transmission element that is actuated by another control shaft.
Another stroke-variable valve drive is known from German patent application DE 10 2011 108 728 A1. An axially movable cam segment with several cams having different stroke contours is actuated by a control shaft that is located inside the camshaft. The cam segments are splined with a base shaft, causing them to be supported in the cylinder head, whereby the cam segment has a sliding bearing surface that interacts with a slide bearing that is supported in the cylinder head. The use of a slide bearing is necessary since—when the cam segments are supported between the cam groups in a plane near the cylinder axis—the installation space for a multi-stage shifting mechanism of the valve stroke is limited.
Another stroke-variable valve actuating mechanism is disclosed in German patent application DE 10 2010 005 790.8. A bearing arranged on an axially movable cam segment is used for axially positioning the cam segment, whereby a movement of the bearing by means of a control shaft brings about the effective shifting of the different cam contours of the cam segment for the valve actuating mechanism.
In the case of multi-stage shifting systems, supporting the camshaft or cam segments close to the cylinder axis causes problems in terms of the installation space available between the valves, so that, as an alternative, the support can be effectuated on the end faces of the cam segments. An example of this is shown in German patent application DE 195 19 048 A1. The cam segment that has the cam groups is widened at its ends faces, and bearing surfaces are created around the circumferential surfaces; these bearing surfaces—together with a sliding surface configured in the cylinder head—form the sliding bearing for the cam segment.
Another bearing for axially movable cam segments on the end faces is shown in German patent applications DE 10 2008 024 876 A1 and DE 10 2009 021 650 A1. Here, the cam segments are provided with sliding surfaces on their circumferential surfaces for which there are sliding surfaces in the cylinder head that form a corresponding counter-bearing. Therefore, the camshaft is supported by means of a sliding bearing formed on the end face of the camshaft segments and on the cylinder head.
German patent application DE 10 2010 047 993 A1 discloses supporting the base shaft on bearing bushings that rotate along and that are splined with the base shaft. Here, the bearing bushings form stop surfaces for cam segments that can be moved axially on the camshaft.